The invention relates to a process for the treatment of a thin brittle metal strip and products obtained from the treatment of the strip, which may comprise forming operations such as a cutting operation. In particular, it relates to a process for obtaining components for magnetic use by cutting them from a metal strip having a nanocrystalline structure.
It has been proposed to manufacture thin strip of a magnetic alloy, and in particular an alloy having a high permeability, which has a structure mainly consisting of very fine grains in an amorphous matrix, the size of which grains may, for example, be between 1 and 100 nm. Such alloys are called nanocrystalline alloys.
Nanocrystalline metallic materials are obtained, in the form of thin strip, for example having a thickness of around 20 μm, from amorphous strip or ribbon produced by casting and rapidly cooling a liquid metal on a cooled roll or between two cooled rolls. The amorphous strip or ribbon is heat treated by holding it at a temperature of around 550° C. for a time of around one hour so that it develops a nanocrystalline structure within a substantial part, for example more than 50%, of its volume.
This heat treatment may be preceded by prior heat treatments at lower temperatures, for example of around 200° C.
When magnetically soft iron-based alloys are cast, cooled and then heat treated, it is possible to obtain, from the strip in the nanocrystalline state, products such as magnetic circuit cores exhibiting excellent magnetic properties which cannot be generally obtained in the case of materials whose structure is different from a nanocrystalline structure.
However, a drawback of strip or ribbon having a nanocrystalline structure is that such strip or ribbon is very brittle so that the slightest mechanical stress results in the strip or ribbon fracturing. It is not even possible to handle the strip or ribbon having a nanocrystalline structure without taking very great precautions, because stresses, even very low stresses induced in the strip result in its undergoing brittle fracture. The only process known at the present time for manufacturing metal components such as magnetic cores from strip having a nanocrystalline structure consists in winding the strip of magnetic alloy in the amorphous state and then heat-treating this strip at a temperature at which the nanocrystalline structure develops. Optionally, the heat treatment may be carried out in a magnetic field in order to modify the hysteresis loop of these nanocrystalline alloys.
It is therefore not possible at the present time to manufacture nanocrystalline magnetic components by mechanical treatment or machining operations including, for example cutting.
It would be of very great interest to obtain magnetic components from strip made of a nanocrystalline magnetic alloy which have a profile whose geometrical shape is well defined. In particular, it would be extremely beneficial to be able to manufacture, from nanocrystalline strip, magnetic components having the shape of washers, U- or E-shapes or even components having complex shapes used for making clocks or watches.
More generally, it may be extremely beneficial to have a process allowing the treatment of a thin brittle metal strip, having a small thickness, generally less than 0.1 mm, the treatment of the strip involving at least one step in which the brittle strip is subjected to stresses, especially cutting or bending stresses.
In EP-0 687 134, it has been proposed to produce a miniaturized transformer comprising a laminated magenetic core consisting of a plurality of thin strips linked together by layers of resin. The laminate is cut to the dimensions of the core to be obtained.
U.S. Pat. No. 4,558,247 describes a saturable core comprising a wound strip made of amorphous magnetic alloy and an insulating film inserted between the successive turns of the core.